Slope failures associated with the floods of September 1987 and February 1988 in Natal and Kwa-Zulu, Republic of South Africa

Abstract

A large number of slope failures occurred as a result of the heavy rains of September 1987 and February 1988 in Natal and Kwa-Zulu. Most of the failures were located to the south and west of the National (N3) road from Mooi River to Durban. Three failures were located adjacent to the N3 road, fifty-five on Provincial roads, 128 along railway lines and thirty-six within the Greater Durban area. Information on the failures was obtained from the relevant authorities and their consulting engineers and engineering geologists. This data was used to classify the failures according to the type of movement, size and material type. Although shales of the Pietermaritzburg Formation are notorious for stability problems, very few slopes on these shales failed during the floods. In the Greater Durban area, most of the failures were classic flows which occurred on over-saturated sandy residual soils on Natal Group sandstone. In the Kwa-Zulu area, slump type failures occurred mostly on steep terrain underlain by granite-gneiss basement rocks. In many areas there is an association between slope failures and dolerite intrusions where typical hummocky ground is indicative of an ongoing history of slope movements. Many slope failures were caused by inadequate drainage and over-saturation of the soil. For example, the majority of failures along the railways resulted from inadequate drainage and poor compaction of fill material. It was therefore concluded that the effect of abnormally heavy rains must be considered in the design of slopes for all types of development.